The semiconductor industry has experienced rapid growth due to improvements in the integration density of a variety of electronic components (e.g., transistors, diodes, resistors, capacitors, etc.). For the most part, this improvement in integration density has come from shrinking the semiconductor process node (e.g., shrink the process node towards the sub-20 nm node). As semiconductor devices are scaled down, new techniques are needed to maintain the electronic components' performance from one generation to the next. For example, low on resistance, low gate charge and high breakdown voltage power transistors are desirable for high power applications.
As semiconductor technologies evolve, metal oxide semiconductor (MOS) transistors have been widely used in today's integrated circuits. MOS transistors are voltage controlled device. When a control voltage is applied to the gate a MOS transistor and the control voltage is greater than the threshold of the MOS transistor, a conductive channel is established between the drain and the source of the MOS transistor. As a result, a current flows between the drain and the source of the MOS transistor. On the other hand, when the control voltage applied to the gate of the MOS transistor is less than the threshold of the MOS transistor, the MOS transistor is turned off accordingly.
MOS transistors may include two major categories. One is n-channel MOS transistors; the other is p-channel MOS transistors. According to the structure difference, MOS transistors can be further divided into two sub-categories, planar MOS transistors and vertical MOS transistors.
Vertical power MOS transistors have widely used in high voltage and current applications due to their low gate drive power, fast switching speed and lower on resistance. In a vertical power MOSFET, the drain and source are placed on opposite sides of a wafer. There may be a trench structure formed between the drain and the source of the vertical power MOS transistors.
Corresponding numerals and symbols in the different figures generally refer to corresponding parts unless otherwise indicated. The figures are drawn to clearly illustrate the relevant aspects of the various embodiments and are not necessarily drawn to scale.